EFFECT OF WATER PH ON THE
CHEMICAL STABILITY OF
PESTICIDES
Pesticides
Fact Sheet
July 2001
Howard M. Deer, Extension Pesticide Specialist
Richard Beard, Extension Equipment and Machinery Specialist
Utah State University, Logan UT 84322-4620
AG/Pesticides/14
Most pesticide formulations such as dry flowables, emulsifiable concentrates and
wettable powders are designed to be diluted with water as the carrier. A water pH higher than 7
which creates alkaline conditions can cause some pesticides to undergo degradation or chemical
breakdown, a process known as hydrolysis. In general, insecticides are much more susceptible to
hydrolysis than are fungicides, herbicides, defoliants or growth regulators. Organophosphate and
carbamate insecticides are more susceptible than chlorinated hydrocarbon insecticides. Some
pyrethroids exhibit susceptibility to hydrolysis.
Tables reporting the pH of water sources across the U.S. list only a few states that have
water with a pH below 7 which is in the acid range. The rest all have sources with varying
degrees of alkalinity. Both surface and ground water supplies usually contain sufficient natural
alkalinity to produce pH levels between 7 and 9.
Some pesticides hydrolyze very rapidly. The hydrolysis rate can be rapid in the pH range
of 8 to 9. For every pH point increase, the rate of hydrolysis will increase by approximately 10
times. The severity of losses due to alkaline hydrolysis is governed by the degree of water
alkalinity, the susceptibility of the pesticide, the amount of time the pesticide is in contact with
the water and the temperature of the mixture.
The solution to the problem is lowering the pH of the water to the optimum range of 4 to
7 before mixing with the pesticide. This is accomplished by adding the recommended amount of
buffering or acidifying agent. It should be noted that the buffering does not affect the residual
activity of the pesticide. The buffering effect starts at the time of mixing, continues during
storage in the tank, and does not stop until the water has evaporated from the spray droplet. Some
materials, such as fixed copper fungicides including basic copper sulfate, copper oxide, and
Bordeaux mixtures, should not be buffered as the acid solution may make the metals soluble and
produce a phytotoxic effect when sprayed on plants. Products used to acidify tank solutions may
be straight acidifying agents or in combination with surfactants or nutrient materials like trace
elements or fertilizer products.
A pH meter is the most accurate method of determining the pH of water. The use of test
papers, such as litmus paper can be non-reliable and can be as much as two pH points in error.
There air liquid color indicators (example: Bromothymol Blue) available which can indicate pH
to within a half point. Water sources, both surface and ground, can and do change in pH with the
passage of time. The change in pH is usually towards a more alkaline condition.
In summary, it is important to know the pH of water used with a pesticide and the
susceptibility of the pesticide to hydrolysis. It is best to mix pesticides just prior to the time of
application and to mix only the quantities that will be used within the shortest time possible. If
conditions dictate, the pH of the water should be adjusted to an optimum level
Common Name
Trade Name
Half-life* at Different pH Values**
2,4-D amine
acephate
azinphos-methyl
bendiocarb
benomyl
captan
carbaryl
carbofuran
chlorothalonil
chlorpyrifos
diazinon
dicamba
dimethoate
disulfoton
fluazifot-P-butyl
malathion
maneb
methomyl
paraquat
pendimethalin
phosmet
propargite
simazine
trichlorfon
trifluralin
Weedar 64
Orthene
Guthion
Turcam
Benlate
Orthocide
Sevin
Furadan
Bravo, Daconil 2787
Dursban, Lorsban
Knox-Out, D.Z.N.
Banvel
Cygon, Dimate
Di-syston
Fusilade
Cythion, Fyfanon
Dithane Manzate
Lannate
Gramoxone Extra
Prowl
Imidan
Omite, 6E & 30W
Princep
Dylox
Treflan
stable at pH4.5-7
pH5 = 40 days, pH7 = 46 days, pH9 = 16 days
pH5 = 17 days, pH7 = 10 days, pH9 = 12 hours
pH5 = 48 days, pH7 = 3 days, pH9 = 45 minutes
pH5 = 80 hours, pH6 = 7 hours, pH7 = 1 hour
pH5 = 32 hours, pH7 = 8 hours, pH8 = 10 minutes
pH7 = 24 days, pH8 = 2.5 days, pH9 = 1 day
pH6 = 8 days, pH9 = 78 hours
stable over wide range of pH
pH5 = 63 days, pH7 = 35 days, pH8 = 1.5 days
pH5 = 14 days, pH7 = 70 days, pH9 = 29 days
stable at pH5-6
pH4 = 20 hours, pH6 = 12 hours, pH9 = 48 minutes
pH5 = 60 hours, pH6 = 32 hours, pH9 = 7.2 hours
pH4.5 = 455 days, pH7 = 147 days, pH9 = 17 days
pH6 = 8 days, pH7 = 3 days, PH8 = 19 hours
pH5 = 20 days, pH7 = 17 hours, pH9 = 34 hours
stable in pH below 7
not stable in pH above 7
stable over a wide range of pH
pH4.5 = 13 days, pH7 = 12 hours, pH8 = 4 hours
effectiveness reduced in pH above 7
pH 4.5 = 20 days, pH5 = 96 days, pH9 = 24 days
pH6 = 3.7 days, pH7 = 6.5 hours, pH8 = 63 minutes
very stable over a wide range pH
* Half-life is the period of time it takes for one-half of the amount of pesticide in the water to
degrade. Each half-life that passes reduces the amount of pesticide present in the water by onehalf, i.e., 1 to 1/2 to 1/4 to 1/8 to 1/16, etc., or 100% to 50% to 25% to 12.5%,etc.
** These values are generalized estimates and reflect trends, but half-life periods may vary
considerably. Hydrolysis depends on other factors besides the pH of the solution, including
temperature, solubility, concentration, type of agitation, humidity, time of day of application,
mixture time in the spray tank, and other pesticides and adjuvants that are in the mixture.
Source: Loveland Industries, Inc. 11/91 and Wilbur-Ellis 3/30/94
PRECAUTIONARY STATEMENT
All pesticides have both benefits and risks. Benefits can be maximized and risks
minimized by reading and following the labeling. Pay close attention to the directions for use and
the precautionary statements. The information on pesticide labels contains both instructions and
limitations. Pesticide labels are legal documents and it is a violation of both federal and state
laws to use a pesticide inconsistent with its labeling. The pesticide applicator is legally
responsible for proper use. Always read and follow the label.
Utah State University Extension and its employees are not responsible for the use, misuse or damage
caused by application or misapplication of the products or information in this publication, and make no
endorsement explicitly or implicitly of this publication or information listed herein.
Utah State University Extension is an affirmative action/equal employment opportunity employer and
educational organization. We offer our programs to persons regardless of race, color, national origin, sex,
religion, age or disability.
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation
with the U.S. Department of Agriculture, Jack Payne, Vice-President and Director, Cooperative Extension
Service, Utah State University, Logan, Utah. (EP/DF/07-2001)